1. Field of the Invention
The present invention relates to a film forming, photosensitive, heat-resistant resin composition and a process for the formation of a heat-resistant insulating resin pattern using such composition. The film formed from the photosensitive, heat-resistant resin composition (in the present specification, the term "film" is used in a broad sense to indicate various coatings, films, thin films, etc.) adheres excellently to base materials and has good heat resistance, moisture resistance, insulating properties, toughness, abrasion resistance, and weather resistance, and further, can be formed at a low cost. Further, the process for formation of a pattern according to the present invention is advantageous in that the use of a photoresist for masking is not necessary, and a pattern of heat-resistant insulating resin film formed by this process is useful as one of the components of a circuit board or a semiconductor device, and can be utilized, for example, as a protective film or an insulating film (layer insulating film) in printed circuits, printed boards, wiring boards, and electronic components for high-density mounting. The term "base material" or "base material to be treated" as used in the present specification refers to mean general-use substrates including semiconductor substrates, ceramic substrates, metallic substrates, and various layered films and wiring.
2. Description of the Related Art
Solder jointing is utilized for mounting IC chips in printed circuits, printed boards, wiring boards and electronic components including multi-chip modules or the like (as exemplified in FIGS. 1 and 2) for high-density mounting, and accordingly, the insulating films used for the above-described circuit substrates should be able to withstand the heat applied during the soldering. Further, since a large amount of information must be rapidly processed, reductions in the size and increases in the capacity of information processors are required. Semiconductor devices constituting a major part of such processors have been integrated through a reduction in the size of unit elements, thus enabling LSIs and VLSIs to be put to practical use. The quantity of heat (heat value) emitted from a semiconductor device increases with an increase in the integration of unit elements, and in an LSI, the heat value amounts to about 10 W.
Integration is achieved by a multi-layerization of the circuit, and when forming a semiconductor integrated circuit element, insulators are needed for layer insulation and surface protection, and inorganic insulators, such as phosphosilicate glass (abbreviated to "PSG"), silicon dioxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4), are used for this purpose.
Although the above-described inorganic insulators have excellent properties such as dielectric strength and heat resistance, it is difficult to form an insulating film having a large thickness therefrom because inorganic insulating films are formed by a chemical vapor deposition process (abbreviated to "CVD").
The surfaces of semiconductor substrates to be coated with an insulating film have a significant number of fine uneven portions having a large aspect ratio, and inorganic insulating films take on a form which is similar to the form of the surface of the substrate, and this raises the problems of impossibility of smoothing the substrate surface and insufficient coverage.
Accordingly, investigations have been made into the use of polyimide, which can be coated by spin coating and has excellent heat resistance, not only as a surface protective film of a semiconductor integrated circuit element but also as a layer insulating film. A polyimide film for use as a surface protective film or a layer insulating film can be formed by dissolving a polyimide precursor, such as bismaleimide, polyamide acid or diamine, in a solvent, such as N-methyl-2-pyrrolidone (abbreviated to "NMP"), coating the resulting solution on a semiconductor substrate by a method such as spin coating, and heating the coating to a temperature of 150 to 400.degree. C. to cause a cyclodehydration reaction to occur, and thus cause the coating to be cured. The cyclodehydration reaction is also referred to as a polyimidalization reaction. Since the polyimide used herein has no photosensitivity in itself, a fine polyimide pattern may be formed by coating a photoresist on a polyimide precursor film, forming a resist pattern by photolithography, transferring the resist pattern to the lower layer by wet etching or plasma etching to form a pattern comprising a polyimide precursor, and heat-treating the pattern to cause a cyclodehydration reaction to occur, to thereby form a polyimide pattern.
A polyimide having photosensitivity in itself, which enables a polyimide to be patterned to any form, has been developed and is commercially available from various manufacturers. In this photosensitive polyimide, a photosensitive functional group is incorporated in the molecule of a polyimide precursor, and photoreaction occurs only in an exposed portion for polymerization, thereby varying the solubility between the unexposed portion and the exposed portion, and development is then conducted by making use of a solvent to dissolve and remove the unexposed portion while leaving only the exposed portion.
In subsequent heat treatment, the photosensitive groups have a poor heat resistance and are thermally decomposed and removed with the advance of cyclodehydration, so that only the polyimide portions having good heat resistance remain. The photosensitive groups can be introduced into the polyimide precursor through a covalent bond, an ionic bond or the like, and various photosensitive polyimides are commercially available.
The use of the above-described photosensitive polyimides, however, have the problem of high cost of the photosensitive polyimide per se, in addition to a large reduction in the film thickness which occurs as a result of the decomposition of a photosensitive group. Further, both non-photosensitive polyimides and photosensitive polyimides have the problem of poor moisture resistance.
For reference, the following patents may be mentioned as examples of patents describing processes for the formation of a pattern.
Japanese Unexamined Patent Publication (Kokai) No. 56-22428 discloses a process for the formation of a polyimide pattern, characterized by comprising the steps of (1) forming a film of a photosensitive polyimide precursor on a substrate; (2) irradiating the film with light in a pattern form and conducting development of the pattern; (3) heating the developed polyimide precursor pattern to convert the pattern to a polyimide pattern; and (4) treating the resultant polyimide pattern with an etchant for polyimide. A polyimide precursor bonded to a photosensitive group through a chemical bond or mixed with a photosensitive compound (such as bichromate) is used as the photosensitive polyimide precursor. In this process, the development residue of unexposed portions can be easily removed, so that through-holes free from defects can be obtained.
Japanese Unexamined Patent Publication (Kokai) No. 59-107346 discloses a heat-resistant photosensitive material comprising a photosensitive polyimide precursor including structural units represented by the following formula: ##STR1## wherein R.sub.1 and R.sub.2 are each an aromatic ring group, R.sub.3 is a thietane ring, and COOR.sub.3 is bonded to the amide group at its ortho-position. An insulating, heat-resistant polyimide pattern having good adhesion to the semiconductor substrate can be prepared through the use of such photosensitive material in the formation of a pattern.
In addition to these Japanese Kokais, although it is not considered to be relevant to the present invention, Japanese Unexamined Patent Publication (Kokai) No. 54-109828 discloses a heat-resistant photoresist composition which comprises 100 parts by weight of at least one polymer selected from a variety of organic polar solvent-soluble, heat-resistant polymers including polyimide, and 0.1 to 100 parts by weight, preferably 1 to 50 parts by weight of a monomeric compound containing at least two ethylenically unsaturated double bonds in its molecule. The amount of the monomeric compound incorporated into the heat-resistant polymer should not exceed 100 parts by weight, because larger amounts cause a reduction of the heat resistance of the resulting photoresist. This is because the specified monomeric compounds do not exhibit excellent heat resistance after curing of the resist.
The use of photosensitive polyimides as layer-insulating films in the production of integrated circuits wherein a considerable amount of heat is generated during use is desired in the art. As described above, photosensitive polyimides, however, have the problem of lowering the precision of the pattern due to the reduction in the film thickness resulting from the decomposition of the photosensitive groups during heat-treating of the photosensitive polyimide precursor to convert it into a polyimide, and further, photosensitive polyimides have the problem of the high cost of the material per se.